Comparison of severity assessment methods for predicting yield loss to Rhizoctonia foliar blight in soybean

Rhizoctonia foliar blight, caused by Rhizoctonia solani Kuhn anastomosis group 1, causes rapid and severe destruction of soybean foliage and pods during hot, wet weather. The objectives of this study were to determine the yield components affected by this disease and whether rating pod damage or foliar damage provides a more reliable method of assessing disease severity relative to impact on yield. Disease severity in the moderately resistant cv. NK S57-11 and the susceptible cv. Buckshot 723 was assessed in field plots in 1996 and 1997 using foliar and pod ratings (0-to-10 scale corresponding to 0 to 100% of tissue affected). Based on results from regression analysis, pod number, seed number, and seed weight per plot decreased as disease severity increased, whereas the proportion of partially filled pods and the weight of 100 seed were not affected. Yield loss appeared to be due primarily to loss of entire pods. Foliar and pod assessments of disease severity correlated positively in 1996 (r = 0.8343) and 1997 (r = 0.5958) for both cultivars, which suggests that either method can be used to identify relative differences among cultivars. However, pod assessments accounted for more variability than foliar assessments under low-disease conditions. Plants exhibiting moderate to severe symptoms of Rhizoctonia foliar blight also retained green stems and pods at harvest, which was evidence of delayed maturity.